Various kinds of conventional screws are widely known, such as shown in FIGS. 1-4. Those shown in FIGS. 1 and 2 have their ends formed with a multi-angled cone A or a cutting edge B so as to cut wood in their rotation. The FIGS. 3 and 4 show screws having their ends formed with a lengthwise groove C or a lengthwise cutting groove D.
However, those four kinds of conventional screws do not have good function in being driven in rather hard wood, still having drawbacks of breaking of screws and rippings in an object wood during driving action.
A known conventional screw disclosed in U.S. Pat. No. 3,492,908 has scrap grooves as shown in FIGS. 3, 4 and 5. The scrap grooves 13 are shaped curved and the same depth at a front and at the rear end, and there are three of them evenly provided from the bottom end. Each scrap groove 13 has two fifths of the length of the rod, as shown in FIG. 1, so when the screw of this patent runs in longer than the length of the scrap grooves 13, some wood scrap may not be able to be removed out, and possible to let the wood split, lowering the strength of the wood structure.
U.S. Pat. Nos. 2,278,411 and 951,437 have the nearly same structure, having the same advantage and the same disadvantage as U.S. Pat. No. 3,492,908, not described here.
Next, U.S. Pat. No. 1,465,148 has scrap grooves 7, 8 and 9 of triangle of a right angle, not easily made by wrought iron process as U.S. Pat. No. 2,278,411. In addition, the scrap grooves of the triangular right angled shape has a cutting function larger than the scrap removing function. Then the threads 14 for engaging wood are formed intermittently due to the scrap grooves 7, 8 and 9 extending from the bottm end to the head 5, decreased in its engaging force.
U.S. Pat. No. 4,673,323 has a scrap groove 18 of the same shape, a triangle with a right angle, and the difference between the two patents is the scrap grooves having different length only.
U.S. Pat. No. 877,131 has scrap grooves of a blade shape for cutting, with the same disadvantages as U.S. Pat. No. 1,465,148. Though the scrap grooves are plural, and formed spiral to extend from the bottm end to the head, they have insufficient engage force as U.S. Pat. No. 1,465,148.